Microwave phase shifter

ABSTRACT

An electrically rotated half wave plate comprised of four electrically varied obstacle planes, consisting of respective sets of varactor diodes, spaced 45* apart in a circular waveguide propagating circularly polarized microwave energy. The four sets of varactor diodes are synchronously driven in a manner to cause the resultant or effective obstacle plane to continuously rotate between 0* and 360* within the waveguide in response to control voltage inputs applied to the four sets of varactor diodes.

Klein Nov. 11, 1975 l5 l MICROWAVE PHASE SHIFTER I75] lnvemor: Gerald LKlein, Westbur NY. Pru nary Eramurer-JamesW. Lawrence AsszsmmEtumrnen-Manm Nussbaum [73] Assignee: Westinghouse Electric Corporation,Attorney, Agem, or Firm-D Schron Pittsburgh, Pa.

[22] Filed Jan. 10, 1974 [2i] Appl. No.: 432.373 ABSTRACT Publishedunder Trial VOlumal-Y Proms An electrically rotated half wave platecomprised of Program on January 1975 as documem four electrically variedobstacle planes. consisting of B respective sets of varactor diodes.spaced 45 apart in 7 v a circular waveguide propagating circularlypolarized j 333/31 A; 333/l 333/98 R microwave energy. The four sets ofvaractor diodes [5 ll '2" H0113) 1H8; HOlP 9/00 are synchronously drivenin a manner to cause the relsgl held of Search 333l-l 31 sultant oreffective obstacle plane to continuously ro- 333/7 D, 7 R, 98 R 98 S,33. /l6 R 9 R, tate between 0 and 360 within the waveguide in re- 30307/34) sponse to control voltage inputs applied to the four sets ofvaractor diodes. [56] References Cited UNITED STATES PATENTS 14 Claims.5 Drawing Figures 3.790.908 2/l974 Burns 333/3l A vomsr 50 CONTROL U.S.Patent Nov. 11,1975 Sheet10f2 3,919,670

U.S. Patfint Nov. 11, 1975 Sheet 2 of2 3,919,670

SET 32 SET 34 SET 36 SET 38 R.F. PHASE ANGLE SET 34 EFFECTIVE PLATEANGLE 6 MICROWAVE PHASE SHIFTER BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates generally to phase shifters ofelectromagnetic energy and more particularly to an electricallycontrolled phase shifter having birefringent properties.

2. Description of the Prior Art In microwave signal processing systems,a requirement exists for a phase shifting device capable of introducingcontinuous phase changes at rates producing frequency offsets in the lowmegahertz (MHZ) range without degrading the spectral purity of themicrowave signal being processed in the general frequency range of50Hz-5UKH2 from the central spectral line. What is generally desired isto produce a linear phase versus time function extending over hundredsof wavelengths in times the order of I microseconds to an accuracy of:3".

Continuously variable phase changers by means of which the phase of anoutput wave is shifted with respect to the input wave are well knowncomponents in the art of microwave transmission. One such phase changerconsists of a mechanically rotated half wave plate coupled between twoquarter wave plates which act as polarization converters whereinlinearly polarized waves are first converted to circularly polarizedwaves, applied to the rotating half wave section and then reconverted tolinearly polarized waves with the instantaneous phase shift introducedbeing dependent on the instantaneous angular relationship of a thin slabof dielectric material relative to a fixed reference estab' lished bythe input polarization converter. Such apparatus is shown and describedin detail in US Pat. No. 2,858,512 issued to RF. Barnett.

A magnetically controlled ferrite phase shifter is furthermore disclosedin US. Pat. No. 2,787,765, A.G. Fox wherein an electrically controlledrotating magnetic field is applied to a ferromagnetic material in thepresence of the magnetic field is utilized for varying the angularrelationship of the applied field to the polarization of the wave energyfor varying the phase shift in the manner previously described butremoving the limitations imposed by mechanically moving parts.

To achieve this function by use of a microwave digital phase shifterrequires a six bit unit with one-half least significant bit accuracy atall phase states capable of switching between each state in the order of3/5 nanoseconds seconds) including settling time to within one-halfleast significant bit at a peak switching rate in the order of 200MHz.

Still other past attempts to accomplish this type of function have beenlimited to much lower phase shift rates and employed combination ofdigital phase shifting and serrodyne frequency offsetting usingtraveling wave tubes.

SUMMARY Briefly, the present invention is directed to an improved phaseshifter of the type including a half wave plate operable upon circularlypolarized waves wherein the angular relationship of a shunt susceptanceobstacle plane relative to a fixed reference is electrically controlledand rotated by means of the operation of four sets of varactor diodes(voltage variable capacitors) mounted inside of a circular waveguide andbeing spaced every 45 therein and covering a span of 120. The fourvaractor sets are synchronously driven by cyclically varying, e.g..sinusoidal control signals such that the first and third sets havesignals applied which are l out of phase with one another as does thesecond and fourth sets; however. a phase difference of exists betweenadjacent sets. Thus four variable shunt susceptances having a 90relative phase shift between adjacent susceptances are provided insideof the wave guide. Each varactor diode set, moreover, comprises at leastone but preferably a plurality of equally spaced varactor diodes alignedin a linear fashion extending longitudinally within the waveguide andsubstantially parallel to the central axis thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view generallyillustrative of a typical prior art rotary phase shifter utilizing amechanically rotated half wave plate;

FIG. In is a perspective view further illustrative of a prior art halfwave plate including a dielectric element;

FIG. 2 is an axial cross sectional view of the preferred embodiment ofthe subject invention;

FIG. 3 is a partial longitudinal sectional view further illustrative ofthe preferred embodiment of the subject invention shown in FIG. 2;

FIG. 4 is a diagram illustrative of the cyclic capacitance/susceptancevariation of the four varactor sets being illustrative of operation ofthe embodiment shown in FIGS. 2 and 3; and

FIG 5 is a diagram illustrative of the driver control voltage signalpattern applied to the four sets of varactor diodes.

DESCRIPTION OF THE PREFERRED EMBODIMENT Prior to discussing thepreferred embodiments of the subject invention, it is desirable to givefurther discussion to prior art apparatus which is diagrammaticallyillustrated in FIGS. 1 and 2, in order to provide a better understandingof the subject invention when it is subsequently considered inconnection with FIGS. 2 and 3. Referring now to FIG. 1, a mechanicallyrotatable phase shifter is basically comprised of a rotatable half waveplate 10 located between input and output transitions to circularwaveguide which typically converts TE rectangular mode waves to the TEmode and quarter wave plates 12 and 14, respectively, which converts thelinearly polarized TE mode waves into a circularly polarized TE modewaves and vice versa. Reference numerals I6 and I8 refer to the inputand output rectangular waveguide section for coupling linearly polarizedwaves to and from the phase shifter. Intermediate the input rectangularwaveguide section and the first quarter wave plate 12 is the rectangularto circular waveguide transition section 20. Similarly, a secondwaveguide transition section 22 couples the quarter wave plate 14 to theoutput rectangular waveguide section 18.

The quarter wave plate sections 12 and 14 are characterized by circularwaveguide members having respective obstacle planes 21 and 23 comprisedof dielectric slabs positioned 45 with respect to rectangular side wallsof the rectangular waveguide. If for example the thin slab of dielectricmaterial were placed parallel to the electric field E of an incidentlinearly polarized electro-magnetic wave propagating the waveguide. thewave would experience a maximum velocity of retardation. The leastvelocity of retardation would occur when the incident electromagneticwave has its electric field E perpendicular to the dielectric material.Aceordingly. when the incident electric field E. which in a rectangularwaveguide such as shown in FIG. I exists in a plane parallel to theshort walls. intersects the 45 plane of the dielectric slab in thequarter wave plate section 12 two component vectors E and E will emergewith a 90 differential phase shift such as shown in FIG. lu. It can beshown that this emergent wave may be regarded as a circularly polarizedelectric wave. Therefore. a linearly polarized electric wave is changedto a circularly polarized electric wave by traveling through the quarterwave plate section 12. Similarly. if a circularly polarized wave ispassed through the quar ter wave plate section 14. it emerges with alinear polarization having an axis of polarization dependent upon thesense of rotation of the incident circularly polarized wave coming fromthe half wave plate 10.

The rotary half wave plate It] is additionally shown in FIG. Ia andtypically consists of a section of circular waveguide 24 having a slab26 of dielectric material of a length L axially mounted across thediameter of the waveguide. Considering that a linearly polarizedelectric field E is converted to a polarized wave consisting of twocomponents E and E at right angles to one another. the diagram in FIG.la illustrates the condition where the electric field vector E isperpendicular to the slab 26 while the field vector E lies in the planeof the slab. As noted above, the electric field vector E will experiencea minimum retardation while the field E will experience a maximumretardation. Thus. the propagation constant B, for the field vector E isgreater than the propagation constant B for the field Ill vector E. inthe slab 26. Passing of the electric field vectors E and B; through thecircular waveguide 24 having a dielectric slab 26 of a length L can beexpressed as:

E out E1 in eand E out Egin e- Accordingly. the length L of thedielectric slab 26 is chosen toobtain a differential phase change. [3.1.3 L I 180.

Thus. if a circular polarized electric wave is introduced into a sectionof waveguide such as shown in FIG. 1a, it emerges with circularpolarization in an opposite sense. i.e.. from a clockwise to a counterclockwise sense. Further,if the circular waveguide 24 containing theslab 26 is physically rotated by an angle d) relative to a fixedreference such as that provided by the dielectric 21 in the inputquarter wave plate section 12 shown in FIG. I, the emergent circularwave from the circular waveguide 24 will have its phase shifted throughan angle 2d). Further. the passage of the emergent circularly polarizedwave from the half wave plate through the output quarter wave plate 14shown in FIG. I converts the circular polarized wave back into alinearly polarized wave shifted in phase relative to the input linearlypolarized wave. It can be seen, therefore. that the continuous rotationof the rotatable half wave plate shown in FIG. I will provide a smoothcontinuous phase variation 'of 0 to 360 for each V2 revolution of thewaveguide 24. Due to the unavoidable imperfection in the manufacture ofwaveguide components. a rotary phase shift additionally normallyincludes means 28 and 30 in the waveguide transitions 20 and 22 toabsorb the small amount of improperly polarized energy present therein.Typically. these may comprise horizontally oriented film loads. Theabsorption of improperly polarized energy effects energy loss but notforward phase shift. Also when desirable. the dielectric slab elementsmay be contoured to minimize reflections. i

While the operating principle of the rotary phase shifter is welldocumented, for example in the above referenced prior art, one may alsorefer to the treatment given to such devices in the text entitledFoundations for Micrmvuve Engineering. Robert E. Collin. McGraw-Hill.1966 at pages 265-270. inclusive.

The same class of device can be designed for operation in connectionwith coaxial or other TEM mode transmission systems by the use oftransitions to rectangular waveguide or by direct launching ofcircularly polarized waves in circular guides from the coaxial lineemploying a helical coupler. thus eliminating the need for quarter waveplates. The rotary half wave plate. however. still is necessarycomponent.

Proceeding now to the inventive concept contemplated by the subjectinvention. the inherent mechanical limitation in the rotation rate atwhich the phase can be varied is overcome by effecting an electricalrotation of the half wave plate member, thereby realizing phase scanningrates in the megahertz (MHz) range. While the aforementioned Fox patentobtains electrical rotation by generating a rotating magnetic fieldaround a ferrite element and thus provides one means for obtaining fastscanning rates. the present invention is directed to still an improvedcontrol phase shifter having birefringent properties. i.e.. producing180 differential phase shift along two orthogonal principal axes.Accordingly. the present invention is directed to a controlled varactorhalf wave plate. the preferred embodiment of which is shown in FIGS. 2and 3. Directing attention now to the preferred embodiment. four sets32, 34, 36 and 38 of linearly aligned varactor diodes (volt age variablecapacitance diodes) are mounted inside of a section of circularwaveguide such that the four sets of varactor diodes are adapted to formfour obstacle planes 33, 35, 37 and 39 spaced apart inside of thewaveguide. The four sets of varactor diodes are identical inconstruction and number and are axially oriented within the waveguide 40and operate as variable shunt susccptances across the waveguide whenrespectively operated by suitably applied electrical control voltages.

Considering one set of varactor diodes for example the second set 34 asan illustrative example and shown in FIG. 3. a plurality of varactordiodes 42 ,42 42,, have one like electrode terminating in alongitudinally extending metal member 44 which may be. for example. apin or a rod which is attached to but insulated from the waveguide 40 bystand-off elements 43 and 45. A DC return element 46 moreover is eitherconnected to the circular waveguide 40 which would be grounded or wouldbe connected back to a ground return lead 48 associated with a varactorvoltage control driver circuit 50. The other like electrode of theplurality of varactor diodes 42, 42, are probe coupled out of thewaveguide 40 and commonly connect to a bias control voltage supplied bythe driver circuit 50. Varactor diode set 32, for example, would beconnected to the driver circuit 50 by means of a common circuit lead 52while varactor sets 34, 36 and 38 would be connected back to the drivercircuit 50 by means of common circuit leads 54, 56 and 58.

Each plurality (n) of varactor diodes forms N 11-] sections providing alength L. The varactor diodes. moreover. are desirably spaced apart onequarter wavelength N4) of the mean operating frequency of the microwaveenergy propagated through the waveguide 40. The plurality of varactordiodes may consist of equal susceptances or be configured to exhibit ataper wherein the first and last susceptance 42 and 42,, have one halfthe value of the intermediate susceptances. This can easily beaccomplished by proper sclection of the diodes themselves or in anyother known fashion such as by paralleling diodes. Also the same resultcan be effected by suitable biasing techniques well known to thoseskilled in the art. It has also been observed that an odd number ofsections (N 3 to 7) provide the best operating characteristic.

In operation. it is desired that the varactor capacitances beselectively varied generally as shown by the diagram in FIG. 4. It canbe observed that alternate sets of diodes operate as opposing pairs ofsusceptances with a 90 shift between adjacent sets. By applying timerelated. e.g.. sinusoidal control voltages such as shown in FIG. 5, thesusceptance pattern is varied substan tially as shown by FIG. 4resulting in an effective or resultant obstacle plane 60 correspondingto the dielectric plate 26 shown in FIG. 2 which rotates within thewaveguide 40. Moreover, where for example the control voltages appliedto the varactor diode sets vary si nusoidally the effective obstacleplane 60 will exhibit a rotation angle (b of l80 for each cycle of theapplied control voltage. Since a 24) phase shift is provided in a halfwave plate. each complete cycle of the synchronized varactor controlvoltages appearing on circuit leads 52, 54, 56 and 58 will provide 360of phase shift for a circularly polarized wave propagated through thecircular waveguide 40.

It will be observed that the control voltages applied to the first andthird set of varactor diodes 32 and 36 are 180 out of phase with respectto one another. The same may be said for the control voltages applied tothe third and fourth set of varactor diodes 34 and 38. A simplificationin the drive circuit connection to the varactor diodes immediatelybecomes evident by considering varactor diode sets 46 and 36 operated asmutually opposite pairs and varactor diode sets 34 and 38 operated as asecond mutually opposite pair. By selectively choosing one set of diodesin each pair to have inverse characteristics or simply by invertingtheir connections. sets 46 and 36 can be commonly driven by a firstcontrol signal from a driver circuit 50 while the second pair of diodes34 and 36 can be driven by a second control signal 90 out of phase withrespect to the first.

The apparatus thus described comprises a microwave analog phase shifterwhich provides the capability for smooth and continuous phase changeover a 360 range by 90 phase shifted variation of control voltage inputsapplied to four cumulative shunt susceptances spaced 45 in a circularwaveguide. It should also be pointed out that a digital processoremploying control data stored in programmable semiconductor memories inconjunction with available integrated circuit digital to analogconvertors can also be used to obtain the desired phase control. Thepresent invention. moreover. is inherently a molulo 211 phase shifterrequiring no reset since the control inputs for 360 are identical tothose for 0 and since a unique combination of a con- Ill 6 trol voltageinput corresponds to each phase angle be tween 0 and 360.

Having thus described what is a present considered to be the preferredembodiment of the subject invention.

1 claim:

I. A microwave phase shifter. comprising in combination:

a section of circular waveguide adapted to propagate circularlypolarized waves therein:

at least four electrically controlled variable capacitance means.adapted to respectively operate as shunt susceptances. in said waveguidesection being located at substantially the same longitudinal positionintermediate the central axis and the inner wall surface of thewaveguide and further being spaced substantially 45 apart in a planetransverse to the central axis; and

electrical circuit means coupling respective mutually synchronizedcontrol signals to said at least four variable capacitance means forselectively varying the capacitance value thereof and providing avarying resultant shunt susceptance therefrom which generates aneffective rotatable obstacle plane for said circularly polarized waves.thereby imparting a phase shift thereto which is a function of the angleof said obstacle plane relative to a fixed reference.

2. The microwave phase shifter as defined by claim I wherein saidelectrical circuit means coupling respective mutually synchronizedcontrol signals to said four variable capacitance means comprisescircuit means generating four sinusoidal control voltages consecutivelyphased 90 apart. and wherein said four control voltages are respectivelyapplied to said four variable capacitance means.

3. The microwave phase shifter as defined by claim I wherein said atleast four electrically controlled variable capacitance means comprisesfour sets of axially aligned varactor diodes and wherein said electricalcircuit means coupling control signal to said four sets of varactordiode means comprises circuit means for varying the capacitance of eachset of diodes in cyclic fashion such that the variation between adjacentvaractor diode sets is shifted 90.

4. The phase shifter as defined by claim I and additionally includingmicrowave circuit means coupled to one side of said circular waveguidesection for coupling circularly polarized waves thereto; and

second microwave circuit means coupled to the opposite side of saidcircular waveguide section for coupling circularly polarized wavestherefrom.

5. The phase shifter as defined by claim I wherein said at least fourelectrically controlled variable capacitance me ans each comprisesrespective sets of varactor diode means.

6. The phase shifter as defined by claim 5 wherein each set of varactordiode means comprises a plurality of spaced apart varactor diodeslinearly aligned along the longitudinal direction of said circularwaveguide section.

7. The phase shifter as defined by claim 6 wherein said plurality ofvaractor diodes are substantially equally spaced and providing N= nlsections where n is the number of the varactor diodes utilized.

8. The microwave phase shifter as defined by claim 7 wherein N is equalto an odd number.

9. The microwave phase shifter as defined by claim 8 wherein N is an oddnumber including the numbers 3 and 7.

10. The microwave phase shifter as defined hy claim 1 and additionallyincluding first microwave circuit means coupled to one side of saidcircular waveguide section for converting linearly polarized way es tocircularly polarized waves. and

second microwave circuit means coupled to the other side of saidcircular waveguide section for converting circularly polarized waves tolinearly polarized waves.

11. The phase shifter as defined by claim 10 wherein said first andsecond microwave circuit means comprises quarter wave plate microwavecircuit means.

12. The microwave phase shifter as defined hy claim 1] wherein bothquarter wave plate microwave circuit means includes a circular waveguideportion; and

additionally including first and second waveguide transition elementsrespectively coupled to the first quarter wave plate microwave circuitmeans for providing a transition from rectangular waveguide to circularwaveguide. and to said second quarter 8 wave plate circuit means forproviding a transition from circular waveguide to rectangular waveguide.

13. The microwave phase shifter as defined by claim 11 wherein said atleast four electrically controlled variable capacitance means eachcomprises a set of spaced apart axially aligned varactor diodeselectrically connected together in respective parallel circuit relationship.

14. The microwave phase shifter as defined by claim 12 and wherein saidelectrical circuit means coupling respective mutually synchronizedcontrol signals to said four sets of varactor diodes comprises circuitmeans coupling a like cyclically variable control signal to each set ofvaractor diodes with each control signal being phased 90 apart such thatthe first and third set of varactor diodes receive a first pair ofsignals 180 apart and wherein said second and fourth set of varactordiodes receive a second pair of signals 180 apart.

1. A microwave phase shifter, comprising in combination: a section ofcircular waveguide adapted to propagate circularly polarized wavestherein; at least four electrically controlled variable capacitancemeans, adapted to respectively operate as shunt susceptances, in saidwaveguide section being located at substantially the same longitudinalposition intermediate the central axis and the inner wall surface of thewaveguide and further being spaced substantially 45* apart in a planetransverse to the central axis; and electrical circuit means couplingrespective mutually synchronized control signals to said at least fourvariable capacitance means for selectively varying the capacitance valuethereof and providing a varying resultant shunt susceptance therefromwhich geNerates an effective rotatable obstacle plane for saidcircularly polarized waves, thereby imparting a phase shift theretowhich is a function of the angle of said obstacle plane relative to afixed reference.
 2. The microwave phase shifter as defined by claim 1wherein said electrical circuit means coupling respective mutuallysynchronized control signals to said four variable capacitance meanscomprises circuit means generating four sinusoidal control voltagesconsecutively phased 90* apart, and wherein said four control voltagesare respectively applied to said four variable capacitance means.
 3. Themicrowave phase shifter as defined by claim 1 wherein said at least fourelectrically controlled variable capacitance means comprises four setsof axially aligned varactor diodes and wherein said electrical circuitmeans coupling control signal to said four sets of varactor diode meanscomprises circuit means for varying the capacitance of each set ofdiodes in cyclic fashion such that the variation between adjacentvaractor diode sets is shifted 90*.
 4. The phase shifter as defined byclaim 1 and additionally including microwave circuit means coupled toone side of said circular waveguide section for coupling circularlypolarized waves thereto; and second microwave circuit means coupled tothe opposite side of said circular waveguide section for couplingcircularly polarized waves therefrom.
 5. The phase shifter as defined byclaim 1 wherein said at least four electrically controlled variablecapacitance means each comprises respective sets of varactor diodemeans.
 6. The phase shifter as defined by claim 5 wherein each set ofvaractor diode means comprises a plurality of spaced apart varactordiodes linearly aligned along the longitudinal direction of saidcircular waveguide section.
 7. The phase shifter as defined by claim 6wherein said plurality of varactor diodes are substantially equallyspaced and providing N n-1 sections where n is the number of thevaractor diodes utilized.
 8. The microwave phase shifter as defined byclaim 7 wherein N is equal to an odd number.
 9. The microwave phaseshifter as defined by claim 8 wherein N is an odd number including thenumbers 3 and
 7. 10. The microwave phase shifter as defined by claim 1and additionally including first microwave circuit means coupled to oneside of said circular waveguide section for converting linearlypolarized waves to circularly polarized waves; and second microwavecircuit means coupled to the other side of said circular waveguidesection for converting circularly polarized waves to linearly polarizedwaves.
 11. The phase shifter as defined by claim 10 wherein said firstand second microwave circuit means comprises quarter wave platemicrowave circuit means.
 12. The microwave phase shifter as defined byclaim 11 wherein both quarter wave plate microwave circuit meansincludes a circular waveguide portion; and additionally including firstand second waveguide transition elements respectively coupled to thefirst quarter wave plate microwave circuit means for providing atransition from rectangular waveguide to circular waveguide, and to saidsecond quarter wave plate circuit means for providing a transition fromcircular waveguide to rectangular waveguide.
 13. The microwave phaseshifter as defined by claim 11 wherein said at least four electricallycontrolled variable capacitance means each comprises a set of spacedapart axially aligned varactor diodes electrically connected together inrespective parallel circuit relationship.
 14. The microwave phaseshifter as defined by claim 12 and wherein said electrical circuit meanscoupling respective mutually synchronized control signals to said foursets of varactor diodes comprises circuit means coupling a likecyclically variable control signal to each set of varactor diodes witheach control signal being phased 90* apart such that the first and thirdset of varactor diodes receive a first pair of signals 180* apart andwherein said second and fourth set of varactor diodes receive a secondpair of signals 180* apart.